One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disk drive. The most basic parts of a disk drive are a disk that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disk drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disk surface. A microprocessor controls most of the operations of the disk drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within a small ceramic block. The small ceramic block is passed over the disk in transducing relationship with the disk. The transducer can be used to read information representing data from the disk or write information representing data to the disk. When the disk is operating, the disk is usually spinning at a relatively high RPM. These days common rotational speeds are 5100 and 7200 RPM. Rotational speeds of 10,000 RPM and higher are contemplated for the future. At such speeds, the very small ceramic block flies on a very thin layer of gas or air. In operation, the distance between the small ceramic block and the disk is very small. Currently "fly" heights are about 0.0003 mm. In some disk drives, the ceramic block does not fly on a cushion of air but rather passes through a layer of lubricant on the disk.
Information representative of data is stored on the surface of the memory disk. Disk drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disk spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disk is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is either divided, such as sectors, or grouped together on the tracks. In some disk drives, the tracks are a multiplicity of concentric circular tracks. In other disk drives, a continuous spiral is one track on one side of a disk drive. Servo feedback information is used to accurately locate the transducer with respect to a track. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The electrical leads for both data and control signals from the head transducer are generally routed to the head transducer on the surface of the suspension and on the surface of the arm to a flex cable at the base of the arm. The signals are then routed to a preamplifier chip located either on the disk drive electronics card or on the flex cable. The electrical frequency response of the circuit, including the head transducer, the preamplifier and the electrical connection between the two determines the observed sharpness of the magnetic transitions on the disk. The sharper the observation (higher frequency response), the closer the bits can be placed together and still be discretely observed or recorded. Thus, the frequency response of the circuit, including the head transducer, the preamplifier and the electrical connection between the two, is a key factor in determining the bits per inch that can be placed on the media disk. The frequency response of the circuit including the transducer head, the preamplifier and the interconnect will be a limiting factor on the linear density as linear densities (number of bits per inch (BPI)) gets in the range of 300 KBPI to 600 KBPI.
The actuator assembly is composed of many parts that contribute to the performance required to accurately hold the read/write head in the proper position. There are two general types of actuator assemblies, a linear actuator and a rotary actuator. The rotary actuator includes a pivot assembly, an arm, a voice coil yoke assembly and a head gimbal suspension assembly. The rotary actuator assembly pivots or rotates to reposition the transducer head over particular tracks on a disk. A suspension or load beam is part of the head gimbal suspension assembly. The rotary actuator assembly also includes a main body which includes a shaft and bearing about which the rotary actuator assembly pivots. Attached to the main body are one or more arms. One or typically two head gimbal suspension assemblies are attached to the arm. Currently in most head gimbal suspension assemblies, the length of the arm is approximately equal to the length of the suspension. The length of the arm and the length of the suspension determine, in part, the mechanical resonance frequency of the actuator assembly.
One end of the suspension is attached to the actuator arm. The transducer head, also known as a read/write head, is found attached to the other end of the suspension. One end of the actuator arm is coupled to a pivot assembly. The pivot assembly is in turn connected to a voice coil motor attached to a voice coil yoke on the main body of the actuator assembly. The other end of the actuator arm is attached to the head gimbal suspension assembly. The head gimbal suspension assembly includes a gimbal to allow the read/write head to pitch and roll and follow the topography of the imperfect memory disk surface. The head gimbal assembly also restricts motion with respect to the radial and circumferential directions of the memory disk. The suspension assembly is coupled to the actuator arm as part of the main body of the actuator assembly which holds the pivot support and is coupled to the voice coil motor. Currently, the pivot assembly is mounted within an opening in the main body. When a number of arms are attached to the main body, a unitized E-block is formed. The E-block includes the arms for mounting the suspension on one end and a voice coil yoke for the voice coil motor on the other end.
U.S. Pat. No. 5,283,704 issued to Reidenbach illustrates another actuator system composed of individual components instead of the unitized E-block. This actuator system is "built up" from at least one individual actuator arm, spacer rings, a separate voice coil yoke frame assembly, and a separate bearing cartridge. A voice coil is located on the voice coil yoke. The voice coil and magnets attached to the housing of the disk drive form a voice coil motor. The disk drive includes a feedback control loop to enable accurate positioning of the transducer head. The disk drive system produces control signals sent to the voice coil motor to move the actuator arm and the suspension supporting the read/write head across the memory disk in a radial direction to the target track. The control signals indicate to the motor the magnitude and direction of the displacement. The control signals can also be used to maintain the position of the read/write head or transducer over a particular track.
These control and data signals are transmitted from the transducer head to the signal processing preamplifier chip. Currently, discrete wires from the transducer head to the actuator flex circuit or electrical traces fabricated on the suspension leading to the actuator flex (such as HTI, TSA products) or by small flex circuit assemblies that connect the head to the actuator flex (such as Innovex's FAST products) are used to form the interconnection. These interconnects have a characteristic impedance per length that interacts with the preamplifier chip and transducer head to determine the system frequency response. Lower interconnect impedance allows higher circuit frequency response and hence cleaner read signals.
To minimize noise and the inductance of the leads, the preamplifier and write-current sources are usually placed near the actuator arms. Wires or leads are typically strung over the surface of the actuator arm and pass to the preamplifier attached near the actuator arm. The wires are typically twisted in pairs to minimize cross talk between the wires. Cross talk results in noise in the wires. Such noise can produce inaccurate readback signals sent to the preamplifier. Minimizing noise from the preamplifier is critical since noise from the preamplifier will be amplified and may produce dominating noise in the amplifiers which follow in the circuitry of the data channel. Moving the preamplifier as close to the transducer as possible minimizes noise in the leads and minimizes the noise produced in the channel circuit. In addition, moving the chip closer to the transducer improves the frequency response of the head and the preamplifier circuit as a function of the lower interconnect impedance.
In the past, chips have been placed on the arms of disk drives with linear actuators where interdisk spacing, and the weight of the arm were not concerns see U.S. Pat. No. 4,891,723 issued to Brian Zak on Jan. 2, 1990. Placing the chip on the thin stainless steel arms or suspension load beams associated with today's disk drives with rotary actuators has significant difficulties. The preamplifier chip produces large amounts of heat. The heat produced cannot be dissipated by the thin, stainless steel actuator arm or suspension used in actuators of current disk drives. A chip could be placed on thick aluminum arms or E blocks to provide the arms with the ability to carry heat away from the chip. However, the benefit would be minimal since the head and transducer would still be 25 mm or more away from the preamplifier chip. Some current disk drive designs have the chip mounted in the flex cable attaching to the base of the arm, so moving the chip to the end of the arm using conventional arm and suspension lengths would also yield minimal benefit.
Actuator arms act as spring-mass-damper systems and have resonant frequencies that can degrade the performance of the servo system. Every closed loop servo motor system has a predetermined bandwidth in which mechanical resonances occurring within the bandwidth degrade the performance of the servo motor system. The actuator arm is one key source of unwanted mechanical resonances. Accordingly, the bandwidths of most servo motor systems are designed so that resonances of the actuator arm and suspension occur outside the bandwidth. Each actuator arm has a unique resonance characteristic. Current actuator arms are made of stainless steel, aluminum or magnesium. Suspensions are typically made of stainless steel.
The resonance characteristics of the arm has bending modes and torsion modes with frequencies that are within the same frequency range as the suspension and the magnetic storage disk (1 kHz to 8 kHz). Great care must be used when designing an actuator system to prevent alignment of resonance modes that would create very high gains and an unstable servo performance. Alignment of resonance modes means one component resonates at a frequency which is very near or the same as the resonant frequency of another component.
Stainless steel or aluminum arms could be made thicker to increase the bending and torsion mode frequencies, but the greater mass significantly degrades the performance of the actuator assembly by increasing the moment of inertia of the arm. Inertial increase will decrease the access time for moving the transducer between data tracks. One constant goal of disk drive design is to reduce access times. Yet another problem is the increase in the current requirements necessary to move the voice coil motor. Increased current results in increased heat within the disk enclosure and increased power requirements.
Use of a thicker steel will also result in other problems. For example, a higher mass assembly will cause significant degradation of shock resistance of the disk drive system. Higher mass assemblies also imply less stability in the form of head lift-off. When a large shock impulse in the vertical direction is applied to the actuator arm, the head gimbal assembly "lifts off" and slaps back on the disk surface. This head slap damages the surface of the disk.
Other metals such as aluminum have been used in making the arm, but the key parameter determining the resonance characteristics of a fixed geometry actuator arm is the stiffness-to-mass ratio of the material, which is about he same for aluminum and stainless steel. Of currently available materials that have been used as actuator arms, only beryllium alloys, ceramics, and carbon composites have significantly higher stiffness-to-mass ratios over that of currently used stainless steel or aluminum.
The demand for higher track density increases steadily as demand for increased storage capacity grows. As a result, increasing the performance of the actuator assembly by increasing the resonance frequencies of the arm, the suspension and the entire actuator assembly is a requirement for future systems. There is also a need for a disk drive system with lower access times. There is also a need for disk drives with actuator arms having a lower moment of inertia. There is still a further need for an arm that has a high stiffness-to-mass ratio such that the length of the arm can be extended to allow the placement of a preamplifier chip close to the read/write transducer.
There is also a need for faster data channels with less noise. Furthermore, there is a need for data channels with lower read error rates. There is also always a need for a more clear signal to increase the speed and reliability of the channel and increase the integrity and density of the data stored on the disk. If the signal is easier to read, the data retrieval process may be able to be conducted more quickly with less need for error correction codes and error correction procedures.